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rebol[ title: "lexigraphic permutations" author: "Tom Conlin" date: 10-Oct-2003 file: %lexpem.r purpose: {to generate permutations of a series in the order they would be found in a dictionary} example: {last lexperm "belor" 104} Library: [ level: 'intermediate platform: 'all type: [function tool] domain: [text math] tested-under: none support: none license: none see-also: none ] ] lexperm: func [{I think I first saw this algo in the TOMS fortran library @ ACM} perm [series!] "series to be permuted" n [integer!] {Natural number of permutations to attempt, stops short if sorted order is reached first } /local lo hi result tmp swap ][ if not positive? n [return none] swap: func [a [series!] b [series!] /local t][ if not equal? a b [t: pick a 1 change a pick b 1 change b t ]] result: make block! n insert result copy perm for i 2 n 1 [ hi: back tail perm lo: back hi ; while suffix decending -- seek from tail towards head while[all[not head? lo greater? pick lo 1 pick (next lo) 1 ]][lo: back lo] ; lo points to mis-ordered item nearest to tail (pivot) ; set hi to first item from tail not in order w.r.t. the (pivot) item ; either exists or we have searched to (pivot) while[greater? pick lo 1 pick hi 1][hi: back hi] ;if no mis-ordered item exists we are done if equal? hi lo [break] ; otherwise exchange dis-ordered items swap lo hi ; reverse all items ABOVE but not including the (pivot) hi: back tail perm lo: next lo while[lesser? index? lo index? hi][ swap lo hi lo: next lo hi: back hi ] insert tail result copy perm ] result ]
halt ;; to terminate script if DO'ne from webpage